1. Field of the Invention
The present invention relates to an electrical device using a battery and also to a battery pack.
2. Description of the Prior Art
Batteries have been used widely as a power source for electrical devices.
Examples of the electrical devices using such a battery as power source include those in the structure having a compartment removably storing a primary battery, those having a chargeable secondary battery in the main body, and the like.
In the tread toward improvement in performance, power consumption of portable electrical devices such as cellphone and digital camera is also increasing gradually. Accordingly, the main power source for such a high-performance portable electrical device is preferably larger in capacity and higher in energy density. The main power sources for portable electrical devices are mainly secondary batteries, but such a secondary battery should be charged with a battery charger or with a power supply device called AC adapter that converts commercial AC power supply voltage into DC charging voltage, as it is connected to commercial AC power source, and thus, use of it outdoor, where the secondary battery cannot be charged, is inconvenient. Recently, a system of charging a secondary battery by using an auxiliary power source that can be removably installed in such a portable electrical device was reported. A primary battery is used as the auxiliary power source.
However, the primary or secondary battery used as main power source and the primary battery used as auxiliary power source in the electrical device described above may be discharged by leak current into an excessive discharge state if they are left connected to the circuit of the electrical device. These batteries have a problem that they may be degraded, causing leakage and drastic temperature rise of the cell in the excessive discharge state.
FIGS. 6A and 6B are graphs showing the change of the output voltage and the electric potential of the electrode (electric potential vs. HgO/Hg) when an alkaline battery, an example of the primary battery, is discharged continuously at a constant current. FIG. 6A shows the change of the alkaline-battery output voltage, while FIG. 6B shows the change of the electric potential of the positive and negative electrodes of the alkaline battery.
In the case of an alkaline battery, a manganese electrode is used as the positive electrode, and a zinc electrode as the negative electrode. The difference between the electric potential of the manganese electrode and that of the zinc electrode shown in FIG. 6B corresponds to the alkaline battery output voltage shown in FIG. 6A. Along with progress of discharge of the alkaline battery, the electric potential of the manganese electrode declines gradually and the electric potential of the zinc electrode rises gradually, leading to decrease in output voltage, and, when the output voltage reaches a discharge terminating voltage, the lowest voltage allowing reliable discharge, the electric potential of the zinc electrode rises beyond the hydrogen gas generating electric potential, resulting in generation of hydrogen gas from the zinc electrode.
When the alkaline battery is discharged continuously further, the electric potential of the manganese electrode declines gradually, while the voltage of the zinc electrode rises rapidly, while hydrogen gas is generated continuously form the zinc electrode. The electric potential of the manganese electrode is stabilized in a particular range of −0.4 to −0.3 V, but the electric potential of the zinc electrode rises further until the electric potential of the zinc electrode becomes higher that that of the manganese electrode; i.e., the system becomes in a so-called polarity reversal state in which the manganese electrode becomes a negative electrode, while the zinc electrode a positive electrode; and thus, the alkaline battery then has a negative output voltage. Because the electric potential of the manganese electrode (negative electrode) is then higher than the hydrogen gas generating electric potential, hydrogen gas is generated from the manganese electrode.
When an alkaline battery is discharged continuously by leakage current of circuit as described above, hydrogen gas may be generated in the battery, leading to increase in pressure of the battery and thus easier leakage of the electrolyte solution.
FIG. 7 is a graph showing the change of the output voltage of a lithium battery, an example of primary battery, when discharged continuously at a constant current. In the case of a lithium battery, a manganese electrode is used as the positive electrode, while a lithium electrode as the negative electrode. Along with progress of discharge of the lithium battery, the output voltage declines gradually. During the period of the battery output voltage declining to 0 V, the lithium electrode may be dissolved, but no lithium dendrite deposits on the manganese electrode.
When the battery output voltage becomes lower than 0 V and the polarity reversal occurs, lithium dendrite deposits on the manganese electrode, and lithium is dissolved and deposited both on the lithium and manganese electrodes. The lithium solubilization/precipitation reaction reaches equilibrium between the lithium and manganese electrodes, and the battery voltage becomes constant temporarily. In such a case, after the polarity reversal, the precipitated lithium dendrite penetrates through the separator, causing short circuiting of the lithium electrode with the manganese electrode, and the short-circuit current may raise the temperature of the lithium battery rapidly.
Even when the lithium battery is not short-circuited during the battery voltage is constant in the lithium battery in the polarity reversal state, if the lithium battery is discharged continuously, iron (Fe) may be dissolved, for example from the case of the lithium battery or the current collector, causing deposition of iron. The deposited iron then penetrates through the separator, causing short-circuiting of the lithium electrode with the manganese electrode, and the short-circuit current may raise the temperature of the lithium battery rapidly.
As described above, a battery in the excessive discharge state may cause troubles such as leakage of its solution and rapid temperature rise, and thus, various methods were proposed to solve the problems. For example, methods of preventing excessive discharge of battery by installing an excessive discharge prohibiting circuit in battery are reported (see for example, Japanese Patent Unexamined Publication No. 2002-525806(kohyo)). Alternatively, methods of absorbing the hydrogen gas generated in battery are proposed for preventing the leakage by excessive discharge of the alkaline battery, and specific examples thereof include methods of using a hydrogen-adsorbing substance placed on the internal face of the alkaline battery casing (see, for example, Japanese Patent Unexamined Publication No. 07-272770) and methods of using a hydrogen adsorption catalyst placed, for example, in the alkaline battery (see, for example, Japanese Patent Unexamined Publication No. 2004-502280(kohyo)).
However, as described in Japanese Unexamined Patent Publication No. 2002-525806(kohyo), the battery containing an excessive discharge prohibiting circuit by the method of preventing excessive discharge by using an excessive discharge prohibiting circuit is indeed a high-resistance resistor, but there remained a possibility of a trace amount of current flowing in the battery. Accordingly when multiple batteries connected to each other in series are used, if any one of the batteries in series is left in the state where the excessive discharge prohibiting circuit is in operation, the battery may be discharged continuously, as driven by the other batteries still retaining some capacitance.
Similarly to the methods described in Japanese Patent Unexamined Publication Nos. 07-272770 and 2004-502280(kohyo), the method of using an absorbent absorbing the hydrogen gas generated in the alkaline battery had a problem that pressure rise and leakage of the battery occur by the unabsorbed hydrogen gas because, if hydrogen gas is generated in an amount more than the hydrogen-absorbing capacity of the hydrogen-adsorbing substance, it is not possible to absorb the hydrogen gas completely.